1. Field of the Invention
This invention relates to photo-exposure systems which are used to produce semiconductor devices. These systems operate by successively exposing circuit patterns in an overlapping fashion onto a semiconductor wafer. More particularly, the present invention relates to a step and repeat photo-exposure system in which a pattern which is contained on an exposure mask is exposed by a camera onto a semiconductor wafar in a reduced fashion. The wafer is stepped and repeated in order to position different die sites beneath the camera of the photo-exposure system. The step and repeat operation continues until all of the die sites on the wafer have been exposed. A new mask is then used to expose a second circuit pattern in an overlapping relation to the intial circuit pattern formed at each die site.
Typically, several hundred individual dies may be formed on a single wafer. In order to insure that the integrated circuits formed by means of the exposure of overlapping circuit patterns operates properly, it is critical to expose the successive patterns in precise overlapping alignment. During the exposure process, an image of the circuit pattern on the exposure mask is formed on the wafer by means of a reduction lens system. If fine alignment and high resolution are to be achieved, the image which is formed by the reduction lens must be precisely focused. The present invention is directed to a system for achieving such precise focus.
2. Description of the Prior Art
In order to achieve precise focus, it is necessary to maintain the surface of the wafer a predetermined exact distance from the reduction lens. If the wafer were precisely flat, this requirement would not present any significant problems. However, wafers may not be precisely flat, i.e., they may be very slightly wedge shaped. In addition, a wafer undergoes process steps in between each circuit pattern exposure, and such process steps may cause the wafer to warp, thus causing its surface to be uneven. The variation in the flatness of the surface of the wafer is generally extremely small; however, because of the very small dimensions of the elements of the circuit patterns, even minute variations in focus caused by a lack of flatness can adversely effect the operation of the system.
One method of achieving exact focus is disclosed in U.S. patent application Ser. No. 038,349, filed on May 11, 1979, entitled "SINGLE LENS REPEATER" and assigned to TRE Corporation, the same assignee as the present invention. In this system, an air jet leveling and focusing system is utilized. The device utlizes a plurality of air jets, typically three, located in the camera housing which carries the reduction lens. The semiconductor wafer is carried on a table which is supported by means of a spherical air bearing, and the air jets supply air under pressure to the surface of the wafer to exert a force on the wafer and the underlying platform. If the wafer surface is not parallel with the housing from which the air jets emanate, the force exerted on the wafer by the individual air jets will not be equal. The unequal force causes the wafer and platform to move with respect to the spherical air bearing support until a condition of equilibrium is achieved. The pressure from the jets causes the surface of the wafer to become parallel to the lower surface of the housing which contains the reduction lens.
The air jets which are used to achieve the leveling of the semiconductor wafer are also used to determine the focus of the system. To this end, the housing containing the reduction lens is moved toward the wafer while monitoring the back-pressure level resulting from the air jets. When the back-pressure reaches a predetermined level, the lens is in the desired position to achieve correct focus.
In order to provide the leveling function, the air jets must be spaced from each other by a sufficient amount to exert a force which will cause the support platform to move within the spherical air bearing. Because of this requirement, the air jets are separated from the exposure location of the camera. As a result of this separation, exact focus may not be achieved despite the detection of the desired back-pressure of the air jets. This is due to the fact that variations in the surface flatness of the wafer may result in the exposure location being at a different distance from the camera than are the air jets. Furthermore, even the small area of the exposure location itself may be warped, and the system described above does not take such warpage into account.